armcm_irq: Move ARM Cortex-M irq handling to new file

The irq handling in sam3x8e isn't specific to the sam3x8e proccessor -
it's generic for all armcm type machines.  So, move the definitions
into a new file generic/armcm-irq.c

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/gpio.c

@@ -6,10 +6,10 @@
 
 #include <stdint.h> // uint32_t
 #include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "board/irq.h" // irq_save
 #include "command.h" // shutdown
 #include "compiler.h" // ARRAY_SIZE
 #include "gpio.h" // gpio_out_setup
-#include "irq.h" // irq_save
 #include "sam3x8e.h" // Pio
 #include "sched.h" // sched_shutdown